Until it becomes apparent otherwise, the discount for Kindle versions of academic texts to date is generally very nominal.
A text of mine is $111 in a hardback book and $89 in a Kindle version. Granted it's slightly larger than 10% discount, but if a student pays $89 for an electronic version that is non-transferrable to be read on a $500 device, versus paying $111 and being able to get at least $60 back reselling the book, I know what I would advise the student to do.
I just looked up a random textbook of mine. New, it sells for $102. On the Kindle, it sells for $43. Does that make your point moot?